On-frequency repeaters are known in the art, for amplifying an input signal without otherwise altering its frequency spectrum. In some cases, an on-frequency repeater may also employ various types of active circuitry in order to enhance the signal-to-noise (S/N) ratio, in addition to simply increasing the power level. A typical application of on-frequency repeaters is for improving wireless services within defined regions of a wireless network, where signal levels would otherwise be too low for satisfactory quality of service. For example, within a building, or a built-up urban area, signal attenuation, shadowing by buildings and/or hills; noise generated by various radio frequency sources, and multi-path effects can seriously degrade the quality of desired RF signals. In some cases, a wireless network provider may install a repeater in order to improve service in a region lying at an edge of the coverage area serviced by a base station, thereby effectively extending the reach of the base-station.
On-frequency repeaters are characterized by the fact that the input and output signals (in either the uplink or downlink path directions) have the same frequency. For the purposes of the present invention, the term “on-frequency repeater” shall be understood to refer to any amplifier system that has this characteristic, irrespective of whether the system is used as part of an wireless communications network, or in any other context. The external input signal received by the repeater (e.g. from a base station or a subscriber's wireless communications device—WCD) can be represented by:Se=A·Cos(ωt+m(t))  (1)Where A is the peak amplitude of the external input signal, ω is the carrier frequency and m(t) is the (frequency) modulation applied to the external input signal. In this case, the corresponding output signal radiated by the repeater can be represented by:So=G·A·Cos(ω(t−δ)+m(t−δ))  (2)Where G is the repeater gain and δ is the time delay through the repeater at the carrier frequency ω.
It will be seen that the output signal (So) radiated by the repeater is a replica of the input signal received by the repeater, that has been amplified and subject to a time delay δ due to electrical delays within the repeater. Part of this delay is inherent to the amplification process, but is primarily caused by band-pass filters used in the repeater to prevent the unwanted amplification of signals outside the frequency band of interest. Generally this delay is inversely proportional to the bandwidth of the filters. The repeater gain (G) provides the increase in signal level that makes the repeater useful.
A limitation of on-frequency repeaters is that the output signal (So) can feed back to the repeater input via a so-called “leakage path”. This feedback signal, which is present at the repeater's input antenna, is then:
                    Sf        =                              (                                          G                ·                A                            L                        )                    ·                      Cos            ⁡                          (                                                ω                  ⁡                                      (                                          t                      -                      δ                      -                      Δ                                        )                                                  +                                  m                  ⁡                                      (                                          t                      -                      δ                      -                      Δ                                        )                                                              )                                                          (        3        )            Where L is the signal loss in the feedback path (that is, the antenna isolation), and Δ is the time delay in the feedback path at the carrier frequency ω.
It will be seen that, if the modulation rate is slow compared to
      1          (              δ        +        Δ            )        ,the feedback signal appears as a phase-shifted version of the external input signal (Se). Consequently, as long as
      (                  G        ·        A            L        )    ⁢                1      ,      the resulting input signal (Si) received by the repeater will be the vector sum of the external input signal Se (Equ. 1) and the feedback signal Sf (Equ. 3). The magnitude of the input signal (Si) is a function of both the amplitude of the external input signal (Se) and the feedback signal Sf, and their relative phases. For a repeater system that employs automatic gain control, the magnitude of the output signal (So), and thus the feedback signal (Sf), will be held approximately constant over a wide range of input power. Such a system will remain stable if the feedback signal Sf is always smaller than the input signal (Se).
However, if the system gain (G) becomes too high, so tat Sf≧Se, then signal leakage between the output and input antennas will cause system oscillation. In principle, system stability can be obtained by ensuring that antenna isolation (L) is equal to or greater than the system gain (G). However, in practice, antenna isolation is difficult to predict, and will frequently change over time. Accordingly, conventional on-frequency repealer gain is manually adjusted by a technician to be less than the expected antenna isolation by a significant margin, in order to provide conditional stability in a changing RF environment. This margin significantly decreases the effectiveness of the repeater and yet does not prevent oscillation for all potential scenarios.
Various systems have been proposed for dynamically monitoring antenna isolation to control or prevent repeater oscillation.
For example, U.S. Pat. Nos. 5,125,108 and 5,584,065 disclose methods of removing interfering signals that are present along with a desired communications signal traffic, using a sample of the interfering signal recieved by a separate, auxiliary antenna. In these references, adaptive techniques are employed to adjust the amplitude and phase of the sample so that, when it is combined with the output of the communication system's receiving antenna, the interfering signal is cancelled.
U.S. Pat. No. 4,475,243 describes an apparatus for minimizing the “spillover” signal from the transmitter to the receiver in a repeater. In this reference, the received signal is translated to baseband (i.e., the carrier is removed) for amplification (regeneration), then translated back up to the same carrier frequency (i.e., remodulating a carrier) for retransmission. An “injection signal” based on sampling the regenerated communication signal is used in conjunction with mixing and correlation techniques to isolate the spillover component of the input signal so that it can be removed at an intermediate frequency (IF) stage of the receiver. This system is designed to handle a single communication signal with narrowband analog voice modulation, and thus is not suitable for use with broadband signal traffic carrying multiple parallel communication signals.
Furthermore, in U.S. Pat. Nos. 4,701,935 and 4,789,993, a digital microwave radio repeater is described in which the desired digital signal is a single signal and is regenerated (amplified) at baseband. In these references, the transmitter-to-receiver coupled interference component that appears at baseband is canceled by subtracting an estimated baseband interference signal. The estimated baseband interference signal is produced by means of an equalization technique implemented by transversal filters whose characteristics are adaptively determined.
U.S. Pat. No. 4,383,331 teaches a system in which a “tag”, in the form of one or more side-frequencies, is added to the output signal prior to its retransmission. The detection of the tag in a received input signal allows the power level of the feed back signal to be measured, and this information allows the repeater to subtract out the interference. In principle, this technique could be applied to monitor antenna isolation in a repeater operating in a broadband RF environment. However, it suffers the limitation that the tag must be located in a side-band (i.e., lying above or below the bandwidth of the desired communications signal traffic) in order to avoid interference corrupting the desired communications signal traffic and/or interfering with other network components. Because antenna isolation can vary strongly with frequency, measurements based on side-band “tags” can, at best, provide only an rough approximation of the antenna isolation at the frequencies of the desired communications signal traffic.
U.S. Pat. No. 5,835,848, teaches a repeater in which antenna isolation is determined using a calibration procedure that is executed during periods in which no communications traffic is present. The calibration procedure involves opening a switch to prevent transmission of signals received at the input antenna; transmitting a test (pilot) signal from the output antenna; and then detecting the signal power of the test signal received through the input antenna. With this scheme, the test signal can be transmitted at any desired frequency, so it is possible to measure antenna isolation, as a function of frequency, across the entire operating bandwidth of the communications traffic. However, in order to accomplish this, there must be no communications signal traffic during the calibration procedure. This necessarily requires interruption of the communications signal traffic, which is highly undesirable.
The systems of U.S. Pat. Nos. 4,383,331 and 5,835,848 suffer the further disadvantage that, in most cases, the power level of the received test (pilot or tag) signal will be very low, requiring highly sensitive detection circuitry to successfully monitor. However, this high sensitivity renders the detection circuit vulnerable to radio frequency interference (RFI) emitted by many common electronic devices and/or test signals transmitted by other repeaters. The presence of noise at the same frequency as the test signal can easily render the system incapable of accurately detecting antenna isolation, and in fact may disable the repeater entirely.
Applicant's co-pending U.S. Pat. application Ser. No. 09/919,888 proposes a solution in which a unique bit-sequence is encoded as a signature signal that is transmitted through an output antenna as a low-level fade impressed on a broadband RE signal. The signal received through the input antenna is correlated with the bit-sequence, and the degree of correlation used as an indirect indicator of system stability. Impressing the signature signal onto the broadband RF signal (i.e., the desired communications signal traffic) as a low-level fade allows the system stability to be continuously monitored without interfering with the communications signal traffic of other devices within the network. The use of a unique bit-sequence to generate the signature signal effectively ensures that the system can readily distinguish between noise (both random RFI and test and/a signature signals from other repeaters) and its own signature signal. However, accurate correlation between the received signal and the bit-sequence is computationally intensive. In some cases, a simpler solution is desired, without sacrificing the advantages obtained by the use of a unique signature signal.
Accordingly, a method and system capable of reliably monitoring stability of an on-frequency repeater, at a moderate cost, remains highly desirable.